The present invention resides in a method and apparatus for working on limp sheet material, particularly layups of limp sheet material which are cut by an automatically controlled cutting blade.
Prior art cutting systems which include automatically controlled cutting machines for limp sheet material are shown in U.S. Pat. No. 3,495,492 and U.S. Pat. Application No. 207,873, filed Nov. 18, 1980, now U.S. Pat. No. 4,328,726 having the same assignee as the present invention. Each of these prior art machines employs a vacuum holddown system in the cutting table on which the limp sheet material is positioned for cutting. When vacuum is applied to the material, the material is compressed and held fixedly in position on the table to perform the cutting with greater ease and accuracy.
The limp sheet materials cut on automatically controlled machines include woven and non-woven fabrics, leather, paper, synthetics such as vinyl, plastic, foils, composites and other materials, and frequently the materials are cut in patterns that are arranged in a closely nested array called a "marker" to minimize the amount of material wasted. Generally, a marker of pattern pieces used, for example, to manufacture garments, may have overall dimensions of 6 feet (2 meters) in width and 24 feet (8 meters) or more in length. The pattern pieces are cut in a single operation by laying the sheet material in a multi-ply stack called a layup, and cutting the pattern pieces from the layup. Conveyorized cutting tables having a length less than the overall length of a single layup are commonly used and cut the layup in two or more sequential segments. A first segment is positioned on the work surface of the conveyor table for cutting in a first operation, and then the second segment or "bite" is moved onto the cutting table for cutting while the first segment is removed.
Since substantial energy is required to evacuate the layup of sheet material, particularly after the material has been partially cut by the blade, the prior art cutting machines have employed a zoned cutting table. In a zoned table, vacuum is applied only to a limited portion of the layup where the cutting blade is operating. The cutting carriage supporting the blade controls the application of vacuum to the appropriate portion of the table through a system of valves and chambers within the bed of the table.
While the zoned cutting tables are intended to reduce the loss of vacuum within a layup and to minimize the amount of energy required to hold the sheet material firmly in position during cutting, their construction is complex and expensive, and substantial leakage occurs through the cuts in the material and also through the table bed which is generally made from a porous material such as bristles to prevent damage to the reciprocating cutting blade. Attempts to reduce leakage in addition to zoning the table have included the installation of air impermeable barriers in the otherwise air-permeable bed to stop horizontal flow of air between the active and inactive zones, the placement of an air-impermeable overlay on the layup of limp sheet material and the exposed portions of the bed and the mounting of endless belts of air-impermeable material on top of the layup to cover the holes or kerfs produced in the material by the cutting operation.
Another approach designed to minimize leakage and loss of vacuum through cut material is shown in U.S. Pat. No. 3,742,802. In this patent, two air-impermeable overlays are wound in opposite directions about two spaced and parallel rollers respectively, and the rollers are mounted on the cutting carriage with the cutting blade. The free ends of the overlays are secured to opposite ends of the cutting table so that the overlay material is wound on and off of the rollers in the manner of a roller shade as the cutting carriage moves back and forth over the table while the blade is cutting. In this prior art, the only portion of the layup exposed during cutting is that portion of the material lying in the gap provided between the two spaced rollers to permit the cutting blade to reach the material. In contrast to the sacrificial overlays that are cut by the blade, the rolled overlays in U.S. Pat. No. 3,742,802 are not cut and may be used again and again in many cutting operations.
It is an object of the present invention to provide an automatically controlled cutting machine that has an air-impermeable overlay spread on and retrieved from cut sheet material that is held firmly in position by vacuum during cutting.